Field of the Invention
The invention is in general related to the field of using liquid fuel, in particular liquefied natural gas (LNG), as a marine fuel in one or more engines of a marine vessel or of a corresponding sea-going ship. In particular the invention is related to a valve system of an LNG tank. More particularly the invention relates to the valve system of an LNG tank according to the preamble of independent valve system claim.
In publication CN 2021185170 is disclosed a blowing-down anti-explosion system with double safety valves, which being mainly composed of a tee joint stop valve, a left safety valve and a right safety valve, wherein the left safety valve and the right safety valve are respectively arranged on the tee joint stop valve.
Brief Description of the Related Art
Liquid fuels that are used as marine fuels are typically liquefied natural gas, liquefied petroleum gas or liquid petroleum gas (LPG), methanol, ethylene. Natural gas is in gaseous form in room temperature. In marine vessels that use natural gas as fuel, the natural gas is typically stored onboard in liquid form, and thus this marine fuel is commonly called by its acronym LNG (Liquefied Natural Gas). Natural gas can be kept in liquid form by maintaining its temperature below a boiling point, which is approximately −163° C. LNG system onboard comprises typically a bunkering station, which is the marine vessel's connection with a LNG terminal on shore or with an LNG bunkering barge, insulated pipes for leading LNG to an LNG tank, which is for storage of LNG and a tank room where LNG is controllably evaporated and its distribution to the engine(s) is arranged. The bunkering station comprises typically one bunkering line (LNG line), one return line and one nitrogen purging line with respective control and safety valves and flanges. The return line is needed when the bunkering operation is performed with two connected hoses for returning the possibly evaporated gas to the bunkering terminal or the bunkering barge. The natural gas is delivered to the engines as a gas but stored as a liquid. A tank room or a tank connection space is associated with a storage tank and contains the equipment to convert the liquid into a gas for safe delivery to the engines. The tank room is also considered a ‘secondary barrier’ in case there should be leakage of the LNG, since liquid pipes and valves are inside it.
For LNG fuelled vessels it is required by international regulations to have at least two pressure relief safety valves and a safety valve branch for each pressure relief safety valve for LNG tank, which two pressure relief safety valves need to be fitted with shutoff valves before and after each of the pressure relief safety valves in each of the safety valve branches, for maintenance purposes of the pressure relief safety valves. Thus the shutoff valves need to be interlocked, in a way that it is possible to close only one pressure relief safety valve and its safety valve branch at a time so that at least one pressure relief safety valve is in operation at all times.
In FIG. 1 is shown schematically a simplified example of a valve system of an LNG tank according to prior art, in which an outlet line 21 from the LNG tank 10 is divided into two safety valve branches 23, 24 each comprising a pressure relief safety valve 12, 15 with shutoff valves 7, 9 before and shutoff valves 13, 14 after each of the pressure relief safety valves 12, 15. After the after the pressure relief safety valves 12, 15 located shutoff valves 13, 14 the safety valve branches connect to a common outlet line 22 for ventilation out to ventilation mast. The pressure relief safety valves 12, 15 will open for safety reasons when the pressure of the LNG tank exceeds the set safety limit. In this prior art system the shutoff valves 7, 9; 13, 14 are globe valves and interlocked with key and lock system with free keys FA, FB, FD and trapped keys TB, TC, TD, TE. The keys are mechanical keys for mechanical locks. For shutting off the pressure relief safety valve 12 in the safety valve branch 23 first free key FA is used to release the shutoff valve 7 to be closed and to release trapped key TB to be used for releasing the shutoff valve 13 to be closed and to release trapped key TC, which cannot be used for the other safety valve branch 24. As free key FA is in use the other safety valve branch cannot be closed. When the pressure relief safety valve 12 is ready to be used again the safety valve branch 23 is opened again in using the keys in reverse order. After this the free key FA is released and ready for use for example for closing the other safety valve branch 24 correspondingly. As can be readily understood, the key interlocking system is quite complicated.